2. Lecture no 6 Fundamentals of environmental science

3.  Studies of biological systems proved that populations were controlled by homeostatic mechanisms that maintain their genetic structure.  Two factors are responsible for the introduction of the concept of homeostasis in ecology:  1. It is necessary for ecological models,  2. A homeostasis system is based on the assumption of interrelated components of the internal structure of the biological system.

4.  Homeostasis mechanisms chiefly concern: 1. Matters cycling and energy flow. 2. Protection of production level 3. System structure. the homeostatic mechanisms operate within the biotic structures of an ecosystem which,in turn, are fitted to the range of variability in the environmental factors of a given biotope.

5.  An ecosystem regulates itself. Homeostasis is the dynamic equilibrium among the living members of an ecosystem, and with their ever- changing environmental conditions, such as wind, rainfall, nutrient availability, air quality, and climate.

6.  In mammals and birds especially, members of the same species spend a lot of time and energy harassing each other, chasing each other, fighting (but not hurting each other much), and avoiding each other. This is called Attack-Avoidance behavior.

7.  When a population of mammals explodes its numbers for some reason, we discover an entirely different homeostatic process.  In many species, especially in small consumers such as mice, rabbits and lemmings, over-crowding creates an increasing stress that damages the thyroid gland, which manufactures essential hormones.  When population densities reach a critical point, the stress becomes extreme and much of the population simply drops dead from endocrine (hormone) system damage.

8.  When the prey of predators such as owls is super- abundant, as at the peak of lemming and hare cycles, the predator will have more young. The snowy owl, for example, will lay up to eleven eggs at peak prey abundance where it normally lays three. So the predators become as super- abundant as prey. If a niche opens in an ecosystem, life fills it as quickly as possible. The prey population crashes. The predators have little choice. They can travel, or starve.

9.  Multiple Births Another homeostatic process which regulates population is multiple births. In some species, such as deer, crowding results in single births. When the same species is not crowded, twins are the rule.  Populations adjust themselves to "undercrowding" as well as to overcrowding. If there is a niche (a way to make a living) available in an ecosystem, life will fill it.

10.  Predation can also be seen as a homeostatic process of communities. Predator/prey relationships have an odd element of cooperation in them. Predators tend to kill the weakest members of the prey population, which include animals who are ill, old, or very young.

11.  Predators are unlikely to kill the strongest and fastest prey, who survive to pass along those traits to their offspring. Through natural selection, predation shapes the prey species as much as it shapes the predator. In other words, deer run fast because the slow ones were eaten.  The conclusion is that predation helps regulate the health and stability of the community.  In natural communities, what is good for the community is not always what is good for single lives.

12.  Natural ecosystems are sustainable. That means that they can maintain themselves with some integrity. It does not mean that they remain in a fixed state; nothing can. Sustainable systems can and do change.

13.  The principle of biodiversity is simple:  In any living community, the more different kinds of organisms there are, the stronger that community will be.  In other words, The more variety of life-forms in an ecosystem, or the more differences there are among the inhabitants, the healthier the community will be, and the more adaptable and resilient the community will be.

14.  Monoculture is a managed refusal of diversity.  Monoculture is the practice of growing single crops on a piece of land, often for year after year. Usually that crop is one bred variety of one species.

15.  heavy use of chemical fertilizers  heavy use of chemical poisons (herbicides, insecticides)  increased susceptibility to diseases  low tolerance to stresses of drought or temperature  reduced resistance to insects

16.  crop failures resulting in famines  reduced soil fertility  increased soil erosion  permanent loss of genetic variety in the crop species  increased habitat for “pest” species  reduced habitat for beneficial species

17.  All species in a community interlive (are essentially symbiotic).   Everything is connected to everything else, and all lives are interdependent   In a natural community, all members benefit and all members suffer interdependently. Each has its role.   Cooperation is a force in nature as powerful as competition. A diversity of organisms is a living example of synergy.